- When the system hostname domain changes, update `localhost.<old-domain>` in Postfix `mydestination` to `localhost.<new-domain>`.
- This sits alongside the existing hostname/FQDN updates for Postfix destinations.
Previous behavior:
`save_dns.cgi` only updated Postfix `mydestination` entries that exactly matched:
- the old short hostname, like `host`
- the old FQDN, like `host.old-domain.test`
It did **not** update:
- `localhost.old-domain.test`
So if you changed:
```text
host.old-domain.test
```
to:
```text
host.new-domain.test
```
Postfix could become:
```text
mydestination = host.new-domain.test, host, localhost.old-domain.test
```
After this hunk, it also updates that localhost domain entry:
```text
localhost.old-domain.test
```
to:
```text
localhost.new-domain.test
```
